This invention relates to hydraulic power units of the type which incorporate a cylinder block containing reciprocating pistons which, through pivotally attached slippers, are in operational engagement with a swashplate. Such hydraulic power units may operate as either pumps or motors. Specifically this invention is concerned with an improved piston for use in such hydraulic power units.
Typically, the pistons that are utilized in hydraulic power units of the aforementioned type have been manufactured from a metallic material, usually steel, which is heat treated. The particular material and heat treatment process utilized are selected to provide adequate structural strength and wear properties necessary for appropriate piston function and life. Closed end hollow pistons are shown in U.S. Pat. No. 3,319,575. Fluid conduits extending the length of the piston are shown in both the U.S. Pat. No. 3,319,575 and also in U.S. Pat. No. 3,882,762.
Because the piston interacts dynamically with both the cylinder block and the slipper-to-swashplate connection which present different loading and wear challenges to the piston, it has been determined that the material property requirements of the piston are not necessarily uniform throughout the entire structure of the piston. For this reason, it is advantageous to provide a piston which is made up of more than a single material so that various structural elements of the piston may possess different strength and wear properties in its final form.
Generally, it has been determined that the cylindrical body of the piston which is in reciprocating contact with the bore of the cylinder block requires strength and surface wear properties necessitating the use of hardened medium carbon steel. The end of the piston which is connected to a slipper through a spherical joint is subject to compressive rotary and pivotal motion and does not require the extreme strength and wear properties of the cylindrical body. The spherical end of the piston can provide satisfactory function and life when formed of low carbon steel which is not hardened.
Additionally, it is often advantageous for the spherical end of the piston to remain malleable to crimping of the end to retain the slipper to the piston in an assembled condition.
Some hydraulic pistons used in a cylinder block of a hydrostatic power unit are of solid steel construction. While they are durable, reliable, and inexpensive to make, their relative greater weight imposes limitations on their speed of operation. They also develop more operational frictional forces.
Alternatively such pistons are constructed with a hollow cavity therein to reduce the weight of the solid steel pistons. However, the hollow cavities become filled with oil which is compressible. The resulting compression and de-compression of the oil within the piston cavity during pressure cycles results in inefficient energy consumption and has an adverse affect on the moments of inertia imposed on the swashplate with which the pistons interact.
The shortcomings of the hollow pistons are overcome in part by pistons which fill the hollow cavity with a plastic material which is lightweight and which is not as compressible as oil. However, it is difficult to get reliable material that can endure the harsh environment of the pistons whereupon deterioration of the plastic material is common. Aluminum slugs in the cavity instead of the plastic material are more durable, but they are difficult to retain within the cavity.
Some pistons provide a hollow cavity with a cap on one end to close the cavity. Oil does not dwell within the cavity because a hollow tube extends through the piston and through the cavity to prevent the cavity from becoming filled with oil. The tubes within the cavity are often reinforced by a washer or the like which spans the distance from the outer diameter of the tube to the inner diameter of the cavity. While pistons of this design solve at least some of the problems outlined heretofore, they are very expensive to make.
Therefore, a principal object of this invention is to provide a closed cavity hydraulic piston and a method of making the piston which is economical of manufacture and which overcomes the above-described shortcomings of existing pistons.
More specifically, it is an object of this invention to provide a closed cavity hydraulic piston which has a hollow piston body with a piston insert thereon which are both formed by a metal injection molding process.
A further object of this invention is to provide a closed cavity hydraulic piston and method of manufacture which provides for adhering the insert to the piston body while simultaneously hardening the piston body and leaving the insert malleable for crimping or swaging, or vice versa.
A further object of this invention is to provide a closed cavity hydraulic piston wherein oil conduits are formed separate from the cavity wherein the insert is adhered to the piston body.
A still further object of this invention is to provide a closed cavity hydraulic piston and method of making the same which will essentially eliminate finishing operations through the fabrication of the body and the insert by a metal injection molding process.
A still further object of the invention is to provide a hydraulic piston which is quiet in operation, capable of being operated at high speeds, energy efficient in operation, and inexpensive of manufacture.
It is a further object of this invention to provide a piston that is comprised of more than a single material.
Further, it is an object of this invention to form a piston from multiple parts, each part being of a different metallic material.
It is also an object of this invention to produce a piston which includes a hardened cylindrical body and a malleable spherical end.
It is a further object of this invention to provide a piston which incorporates a conduit traversing the body and the insert as a path to communicate pressurized fluid from the cylinder bore to the slipper and swashplate operative connection.
Additionally, it is an object of this invention to assemble the piston to a slipper for subsequent reciprocal operation in the bore of a cylinder block and operative connection with a swashplate in a hydraulic power unit.
These and other objects will be apparent to those skilled in the art.